Technical Field
The present invention relates to a numerical control device and a method for monitoring a temperature rise of a motor to perform overload shutdown.
Related Art
If a motor continues operation in an overload condition, the temperature of motor winding rises to eventually cause winding burnout. Thus, in conventional numerical control devices, a temperature sensor such as a thermistor is embedded in a motor winding such that when the temperature rises over the maximum allowable temperature, an alarm is issued and the power supply to the motor is shutdown.
As an alternative to the above temperature sensing method using a temperature sensor, an electronic thermal system has also been used to calculate the temperature of the motor winding so as to place the device into an alarm mode and shutdown the power supply to the motor when the temperature rises over the maximum allowable temperature. In the electronic thermal system, a motor temperature is obtained by calculating the amount of heat generation and heat discharge based on a current command value, history of electric current sensed values, or the like (refer to JP H09-261850 A, JP H06-253577 A, and JP 2008-61302 A).
With reference to FIG. 2, a conventional numerical control device is described below. FIG. 2 is a block diagram showing a typical control block of a conventional numerical control device.
First, an NC command generator 1 analyzes a processing program, generates a positional command P* by applying processes such as an interpolation process and an acceleration/deceleration process to the analysis results, and sends the generated positional command P* to a position controller 2. The position controller 2 receives inputs of the positional command P* and a positional feedback P from an encoder 7, and calculates a speed command V* by applying calculation such as proportional (P) control to these inputs. A speed controller 3 receives inputs of the speed command V* calculated above by the position controller 2 and a speed feedback V from the encoder 7. The speed controller 3 applies calculation such as proportional-integral (PI) control to these inputs to calculate a current command I*. A current controller 4 receives inputs of the current command I* and a current feedback I, and calculates an inverter drive command based on these inputs. An inverter circuit 5 operates a motor 6 by applying electric current to the motor 6 in accordance with the inverter drive command. In the case of speed control such as for a main shaft, a speed command V1* (not the positional command P*) is directly sent to the speed controller 3 from the NC command generator 1.
As the temperature of the motor 6, a motor temperature T (output from a temperature sensor disposed with a winding of the motor 6) or a motor temperature TW (calculated using the electronic thermal system as disclosed in JP H09-261850 A by inputting the current command I* or a current feedback I to a temperature predictor 8) is used.
The motor temperature T or the motor temperature TW is input to a temperature comparator 9 for comparison with the maximum allowable temperature Tmax stored in advance in a storage 10. When Tmax≤T or TW, the temperature comparator 9 sends a power supply shutdown command to the current controller 4 to shutdown the power supply to the motor and place the numerical control device in the alarm mode.